In a bowl, whisk together the egg yolks, sugar, cornstarch and salt. In a medium saucepan bring the milk, heavy cream, and vanilla to a boil, stirring constantly to prevent it from scorching. Whisk 1/2 cup of the hot milk mixture into the egg mixture. Pour this egg mixture into the saucepan, constantly whisking it into the hot milk to prevent clumping. Heat the mixture over medium heat until thickened. Do not let it boil. Strain the vanilla sauce through a fine mesh sieve. Serve immediately or let it cool and store covered in the fridge. Will keep for up to 3 days. Can be served cold or reheated to serve warm. Makes about 2 1/2 cups of Vanillesoße.